Burner control system

ABSTRACT

THE PRESENT INVENTION PROVIDES A BURNER CONTROL SYSTEM FOR USE IN A FURNACE OR THE LIKE. THE SYSTEM INCLUDES SWITCHING CIRCUITRY CONTROLLING ENERGIZATION OF A SOLENOID OPERATED FUEL VALVE, TRIAL CIRCUITRY FOR INITIALLY OPENING THE FUEL VALVE AND ESTABLISHING IGNITION AT THE BURNER, AND FLAME SENSING CIRCUITRY WHICH MAINTAINS THE FUEL VALVE OPEN IN RESPONSE TO THE PRESENCE OF FLAME AT THE BURNER. THE SYSTEM IS THERMOSTATICALLY CONTROLLED IN ACCORDANCE WITH THE TEMPERATURE OF THE HEATED MEDIUM.

' Jan. .12, 1971 s.H.,MA' .AvAs| BURNER CONTROL SYSTEM 2 She efs-Sheet 1Filed Dec. s, 195

Fig. I

ATTORNEYS.

- M L vAsl BURNER CONTROL SYSTEgf 2 Sheets-Sheet 2 Filed Dec. 15; .1968

QM F wwg INVENTOR. swam H. MALAVAS/ BY W115, ATTORNEYS.

3,554,680 BURNER CONTROL SYSTEM Stuart H. Malavasi, West Millington,N.J., assignor to Ranco Incorporated Filed Dec. 3, 1968, Ser. No.780,650 Int. Cl. F23n 5 /00' US. Cl. 431-78 20 Claims ABSTRACT OF THEDISCLOSURE The present invention provides a burner control system foruse in a furnace or the like. The system includes switching circuitrycontrolling energization of a solenoid operated fuel valve, trialcircuitry for initially opening the fuel valve and establishing ignitionat the burner, and flame sensing circuitry which maintains the fuelvalve open in response to the presence of flame at the burner. Thesystem is thermostatically controlled in accordance with the temperatureof the heated medium.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to a burner control system and more particularly relates to anelectrical control system for thermostatically governing a fuel valvefor a burner in a furnace or the like, and which operates in a failsafemanner and is self checking for malfunctions.

Theromstatic control systems for mediums heated by burners such ascontrols for gas fired dryers, oven, furnaces, hot water heaters, andthe like, are constructed so that failure of the control results inclosing a fuel supply valve. Such design is referred to as failsafe andis a necessary feature of any system which is suitable for commercialuse, particularly in household applications. Such systems must operatein a substantially failsafe manner in order to insure avoidance ofexplosion or fire due to accumulated unburned fuel.

With the advent of small electronic components which require relativelylittle power to operate, it has been possible to provide electricalthermostatic control of fuel valves by flame monitoring systems, as wellas to ignite fuel at the burner by electrical ignition systems. Thesystems must operate in a substantially failsafe manner in order to beapproved for sale. These systems must fail safely under various testconditions simulating operating conditions and wherein individualcomponents of the system are shorted out or otherwise caused tomalfunction.

The prior art Certain prior art proposals for electrically controlledfuel valves utilizing electronic circuitry have not provided failsafeoperation, the use of semiconductor devices conditons and thus are notcommercially feasible. Furthermore while some portions of prior artsystems have provided failsafe operation, the use of semiconductordevices to control and energization of solenoid operated fuel valves hasbeen such that a malfunction of the semiconductor controlling the fuelvalve can occur without any apparent effect on the system.

Some previously constructed systems have been constructed and arrangedso that if fuel valve controlling semiconductor elements are shortcircuited the system can continue to function in a seemingly normalfashion so long as other failures in the system do not occur. Thus suchsystems have provided no warning of malfunction of the valve controllingelements and the systems can be operated indefinitely even though themalfunction exists. Continued operation of these systems can result indanger- -United States Patent 01 lice 3,554,680 Patented Jan. 12, 1971ous conditions particularly when used in connection with a thermostaticcontrol. For example if the gas supply is for any reason terminated whenthe thermostat is not satisfied, the malfunctioning fuel valvecontrolling elements would not permit the fuel valve to close andunburned fuel will accumulate about the burner when the fuel supply isreinstated.

Additionally prior art systems utilizing electrical flame detection havenot been capable of operating effectively in the presence of aflickering or high impedance flame at the burner. Such systems haveterminated fuel flow in the presence of such flames when a malfunctionhas not occurred.

Furthermore some previously constructed systems have utilized pulsingcircuitry for initiating fuel flow and causing ignition of the fuelduring an ignition trial period. The trial period is controlled by anR-C circuit associated with a semiconductor element. The previouslyconstructed ignition trial circuits have included transistors forconducting pulses during the trial period; however these devices havenot been entirely satisfactory because variations in the strength of thesignal pulses over the period occur and the duration of the trial periodhas been inconsistent.

SUMMARY OF THE INVENTION The present invention provides a thermostaticcontrol system for a medium heated by a burner which operates in asubstantially failsafe manner; permits maintenance of a high impedanceof flickering flame at the burner; provides an ignition trial period ofconsistant length from cycle to cycle; and is self-checking so that anymalfunction in the circuit which tends to falsely indicate the presenceof flame at the burner is discovered during the cycle of operation ofthe system during. which they occur.

Accordingly, a principal object of the present invention is theprovision of a new and improved burner control system utilizingelectrical components organized to insure substantially failsafeoperation of the system and which is self-checking so that anymalfunction of the system tending to falsely indicate the presence offlame at the burner renders the system incapable of operating after thecycle of operation during which the malfuction occurs.

A preferred form of the invention provides a system having ignitiontrial circuitry, and flame monitoring circuitry associated with athermostatic control device so that a medium heated by a burner can bemaintained at a desired temperature by automatically cycling the burner.The flow of fuel to the burner is automatically terminated when flame isabsent from the burner during a heating cycle, or after an unsuccessfultrial for ignition.

The electrical control system also includes switching circuitrycontrolling energization of the valve. The switching circuitry includesa semiconductor switch element rendered conductive to establish anenergization circuit for the valve. A thermostatically operated switchis connected into the valve energization circuit to control energizationof the fuel valve when the semiconductor switching circuitry isconditioned to permit completion of the energization circuit.

The switching circuitry is coupled to the ignition trial circuit whchoperates the switching circuitry to open the fuel valve for a briefinterval subsequent to closing of the thermostatic switch. The ignitiontrial circuitry additionally operates the ignition circuitry so thatfuel emitted from the burner during the ignition trial interval isignited.

flame is not continuously detected throughout a heating cycle, the flamedetecting circuitry causes the switching circuitry to close the fuelvalve.

The system is self-checking so that malfunctioning of the switchingcircuitry disables the system during the cycle of operation when themalfunction occurs. If, because of malfunction, the switching circuitryremains conductive when the thermostatic switch is opened, safetycircuitry connected to the switching circuitry causes the fuel controlvalve to close and prevents further energization of the fuel valvethrough the thermostatic switch when that switch recloses. Furthermore,any malfunction of the system which falsely indicates the presence offlame at the burner results in operation of the safety circuitry asdescribed in the cycle of operation during which the malfunction occurs.

Other objects and advantages of the present invention will becomeapparent from the following detailed description thereof made withreference to the accompanying drawings which form a part of thespecification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration ofa control system embodying the present invention; and

FIG. 2 is a schematic illustration of a portion of the system of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT For purposes of illustration, aportion of a household furnace embodying the present invention isillustrated in FIG. 1. The furnace includes a combustion chamber 11(illustrated by broken lines) which may be of any suitable constructionand which includes an electrically grounded gas burner 12 connected to afuel supplying system. The fuel supplying system includes a gas pipe 13and a solenoid operated fuel control valve 14. The valve 14 isconstructed so that the solenoid 15 is energized to open the valve anddeenergized to close the valve and preventing fuel from flowing to theburner 12. In the preferred construction energization of the solenoid 15fully opens the valve 14 to permit a substantially unthrottled flow offuel to the burner.

Energization of the solenoid 15 is controlled by a burner controlsystem, generally designated 20, which includes flame controllingcircuitry 21, a thermostatic control apparatus 22 and a suitableelectric power supply 23. While the invention is described in referenceto a household gas fired furnace, it is apparent from the followingdescription that a burner control system embodying the present inventioncan be utilized in conjunction with burners of various types ofcombustion equipment.

The power supply 23 includes suitable step-down transformer T1 havingprimary winding T1P adapted for connection to a l17-volt AC powersupply, and a secondary winding TlS constructed to provide 24-volt ACpower through the flame controlling circuitry 21 and the solenoid 15through a suitable fuse 24.

The thermostatic control apparatus 22 is in the form of a wall-type roomthermostat having a control knob K which is manually operated to adjustthe air temperature. The knob K is linked to a switch 25 including amoving contact arm 26 and a stationary contact 27. A thermallyresponsive device 28 including for example, a temperature sensingbimetal is situated in heat exchange relationship with the air in thehousehold. A linkage L connects the moving contact 26 of the switch tothe knob K and to the thermally responsive element 28. The thermallyresponsive element may be of any suitable construction and accordinglyis not described in detail. When the knob K is set to provide a desiredtemperature, the thermally responsive element opens and closes theswitch 25 at the extremes of a relatively narrow temperature range. Whenthe knob K is turned to an off position, the switch 25 is positivelymaintained open and the thermally 4 responsive element is ineffective toclose the switch regardless of the temperature of air in the home.

The furnace is turned on by rotating the knob K from the off position toa desired air temperature setting. The thermally responsive elementcloses the contacts of the switch 25 as a result of rotation of the knobif the temperature is below that called for. Closing of the contacts ofthe switch 25 establishes an energizing circuit for the flame controlcircuitry 21 from the power supply 23 across the terminals A, B of thecircuit 21 through the switch 25. Energization of the circuitry 21energizes the solenoid 15 to open the fuel valve 14 and, simultaneously,ignition circuitry 29 (see FIG. 2) is activated so that igniting arcsare struck through the flow of gas from the burner to establish a flameat the burner. When a flame is established at the burner the circuitry21 detects the presense of the flame and maintains the fuel valve openuntil the thermostat 22 is satisfied. An ignition trial period isprovided so that should the fuel at the burner 12 fail to be ignited thecircuitry 21 automatically deenergizes the solenoid 15 to terminate thefuel flow.

The terminal B is connected to the ground G in the illustratedembodiment and the voltage level at the terminal A alternates betweenvoltage levels which are positive and negative with respect to theterminal B. The circuitry may presently be described in reference topositive and negative half cycles of the power supply and according tothe convention used herein a positive half cycle of the power supply isa half cycle during which the terminal B is positive with respect to theterminal A and a negative half cycle is a half cycle during which theterminal B is negative with respect to the terminal A.

SWITCHING CIRCUITRY FOR THE SOLENOID 15 The energization circuit for thesolenoid 15 is completed from the power supply through the fuse 24, ajunction 30, the windings of the solenoid, a terminal C of the circuitry21, junctions 31, 32 (see 'FIG. 2), the anode and cathode of a siliconcontrolled rectifier SCRl, a junction 33, a diode D1, normally closedcontacts 34, 35 of a switch S1, the terminal D the closed contacts ofthe thermostatic switch 25, and to the treminal A connected to the powersupply. The solenoid 15 is energized by half wave rectified AC power dueto the rectifying effects of the SCRl and diode D1. A flywheel diode D2is connected in parallel with the solenoid 15 between the junction 32and a junction 36 connected to the terminal B so that the solenoid 15remains pulled in during alternate half cycles in which the solenoid 15is not energized from the power supply. It should be pointed out thatthe terminals A, B, and C are connected to the power supply at all timesand only the terminal D is disconnected from the power supply by openingof the switch 25 when the circuit is not functioning.

The SCRl is an element of switching circuitry which controlsenergization of the solenoid 15 when the thermostatic switch is closed.The SCRl is rendered conductive in response to a pulse to its controlelectrode 40 from a silicon controlled rectifier SCR2 which is connectedinto the circuitry to drive the SCRI. The SCR2 is rendered conductive toestablish a circuit from the terminal B through junctions 41-45, aresistor R1, anode and cathode electrodes of the SCR2, a junction 46,resistor R2, and a junction 47 at the voltage level of the terminal A.The control electrode 40 of the SCRl is connected to the cathode of theSCR2 and when the SCR2 is conductive pulses are conducted to the controlelectrode 40 through junctions 46, 48 and a resistor R3. Establishmentof current flow from the control electrode 40 to the cathode of the SCRlrenders the SCRl conductive to energize the solenoid 15. A filtercapacitor 49 is connected in parallel with the SCRI to prevent circuittransients from rendering the SCRl conductive when undesired. TerminalsA, B are connected at all times to the power source T15.

THE IGNITION TRIAL CIRCUITRY The circuitry 21 includes an ignition trialcircuit generally designated at 50 'which is effective to render theSCRl and the SCR2 conductive when the thermostatic switch is intiallyclosed so that the fuel valve 14 is opened to permit flow of fuel to theburner 12 at least during the relatively brief ignition trial interval.The ignition trial circuitry includes a programmable unijunctiontransistor Q1 which is rendered conductive during positive half cyclesof the power supply to provide a pulse to the control electrode 51 ofthe SCR2 so that the SCR2 is rendered conductive to drive the SCRI asdescribed. The pulse conducting circuit for the control electrode 51 ofthe SCR2 is established from the terminal B through junctions 41-43, aresistor R4, junctions 5254, a diode D3, junction 55, resistor R5,junctions 56, 57, 60, the anode and cathode electrodes of the transistorQ1, a junction 61, resistor R6, diode D4, junctions 62, 63 to thecontrol electrode 51 of the SCR2.

A resistor R7 is connected between the junctions 63, 48 across thecontrol and cathode electrodes of the SCR2 to maintain a suitably highvoltage level at the control electrode 51 to insure triggering of theSCR. The resistor R7 additionally provides a current path from the anodeelectrode of the SCR2 through the control electrode 51 and to theterminal A so that rapid voltage rises across the SCR do not render theSCR conductive when undesired.

The transistor Q1 is poled to conduct half wave 60- cycle pulses to theSCR2 so that when the transistor is conductive the SCR2 and the SCRIconduct during positive half cycles of the power supply. The periodduring which the ignition trial circuitry produces the noted pulses isdetermined by an R-C network connected to the control electrode 65 ofthe transistor Q1 which is connected to a junction 67 in the R-Ccircuit. The R-C circuit is established from the terminal B through theresistor R4 and diode D3, the junction 55, a resistor R8, the junction67, a capacitor C1 and to the power supply through the switch S1,terminal D and the thermostatic switch 25. A secondary charging circuitfor the capacitor C1 is established from the junction 55, through theresistor R5, the junction 56, a resistor R10, a junction '66, a resistorR11, a diode D5, the junction 67, the capacitor C1, switch S1, and tothe power supply through the terminal D and the thermostatic switch 25.

The charging circuits establish a voltage level at the junction 67 whichis dependant upon the charge condition of the capacitor C1. As thevoltage level at the junction 67 increases, the voltage level at thejunction 66 increases. Since the junction 66 is connected to the controlelectrode 65 of the transistor Q1 the voltage level at the junction 66controls conduction of the transistor during positive half cycles of thepower supply. The anode of the transistor Q1., the resistor R5, and aresistor R12, are connected in series between the junction and ajunction 70 connected to the power supply through the switch S1,terminal D and thermostat 25. These resistors form a voltage dividerwhich establishes a voltage level at the junction and anode of thetransistor Q1 which is a fixed proportion of the voltage across thejunctions 55, 70.

As previously noted the transistor Q1 is of the type known as aprogrammable unijunction transistor. This transistor operates insomewhat the same manner as an SCR in that the anode-cathode currentdoes not depend on the relative strength of the gating signal. Thetransistor Q1 is rendered conductive when the anode voltage level ispositive with respect to the voltage level at the control electrode by apredetermined amount. As the voltage level at the control electrode ofthe transistor Q1 approaches the anode voltage level the transistor isnot rendered conductive during positive half cycles. Thus as the voltagelevels at the junctions 66, 67 increase the voltage level at the controlelectrode of the transistor Q1 approaches the anode voltage and thetransistor is rendered nonconductive at a predetermined controlelectrode voltage level.

When the switch 25 is initially closed the capacitor C1 begins to chargeduring positive half cycles and produces a voltage level at the controlelectrode 65 which is negative with respect to the voltage level at theanode of the transistor Q1. Accordingly, the transistor Q1 is renderedconductive during positive half cycles. As the capacitor C1 continues tocharge the voltage level at the junction 66 increases gradually and'when the voltage levels at the junctions 60, 66 are are substantiallyequal to the transistor Q1 no longer conducts.

The charging circuits are constructed of components which maintain thevoltage level at the junction 66 sufiiciently negative with respect tothe voltage level at the junction 60 that the transistor Q1 isconductive only during a period of approximately four secondsimmediately following closing of the thermostatic switch 25. The diodeD3 prevents discharging of the capacitor during negative half cycles ofthe power supply.

A resistor R14- is connected between the junction 61 and a junction 76connected to the power supply through the terminal A. The resistor R14stabilizes the voltage at the junction 60 to prevent erratic ignitiontrial periods which might otherwise occur as a result of line voltagechanges. A filter capacitor C2 is connected between the junction 57, anda junction 77 at the control electrode 65 to prevent changes in thevoltage across the anode and control electrodes of the transistor Q1 inresponse to transients.

The transistor Q1 conducts to provide pulses to the SCR2 resulting inthe fuel valve 14 being opened. Additionally, the transistor Q1 operatesthe ignition circuitry 29. More particularly the transistor Q1 operatesthe ignition circuitry 29. More particularly the transistor Q1establishes a pulsing circuit from the junction 61 through a resistorR13, a diode D6, junction 71 and to the control electrode 72 of asilicon controlled rectifier SCR3. The SCR3 is connected into the fuelignition circuitry and when rendered conductive causes the ignitioncircuitry to establish electric arcs between a spark electrode 75 andthe grounded burner 12 through the fuel flowing from the burner (seeFIG. 1). A resistor R15 is connected between the junction 71 and ajunction at the voltage of the terminal A to prevent the SCR3 from beingturned on at high temperatures, when undesired, due to leakage currents.

After the ignition trial circuitry 50' has operated for approximately 4seconds the capacitor C1 is charged sufficiently to cause the voltage atthe junction 66 to approach the voltage level at the junction 60, andthe transistor Q1 is rendered nonconductive.

THE FLAME DETECTING CIRCUITRY Assuming that a flame is established atthe burner during the ignition trial period, the fuel valve 14 ismaintained open by operation of a flame monitoring or detecting circuitgenerally indicated at 79. The flame detect-' ing circuitry relies onthe rectifying effect of flame on electric current to produce signalpulses by way of a pulse transformer T2 to render the SCRl and SCR2conductive and maintain the valve 14 open so long as flame is detectedat the burner.

The flame detecting circuitry 79 includes a filter capacitor C3connected between the terminals B, A through junctions 41, 81, and fullwave voltage regulating circuits. Voltage regulation during positivehalf cycles of the power supply is provided by the resistor R4, a Zenerdiode Z1 having its cathode electrode connected to the junction 52, anda diode D10 having its anode connected to the anode of the Zener diodeZ1 and its cathode connected to a junction 82 at the terminal A. Thepeak voltage applied across the circuitry 21 during positive half cyclesof the 7 power supply is determined by the Zener voltage of the diodeZ1.

During negative half cycles of the power supply, voltage regulation isprovided by a resistor R20, Zener diode Z2, and a diode D11 which areserially connected between the junction 42 at the treminal B and ajunction 83 connected to the terminal A. The cathode electrode of theZener diode Z2 is connected to the resistor R through a junction 84 andthe anode electrode of the Zener diode Z2 is connected to the anodeelectrode of the diode D11. Thus during negative half cycles of thepower supply the peak voltage across the circuitry 21 corresponds to theZener voltage of the Zener diode Z2.

During negative half cycles of the power supply and when a flame ispresent at the burner 12, a charging circuit is established from thejunction 83 through the resistor R20, the junction 84, a junction 85, acapacitor C4, a junction 86, the terminal E, an electrode 87 positionedin the flame and through the flame to the grounded burner 12. Thusduring negative half cycles of the power supply the plate C411 of thecapacitor C4 is charged to a positive state relative to the plate C4b.The capacitor C4 is maintained in its charged state during positive halfcycles of the power supply due to the rectifying effect of the flame,i.e., the flame does not conduct during positive half cycles.

The capacitor C4 continues to charge through the flame during negativehalf cycles of the power supply until the voltage across the capacitorC4 reaches a level which is sufificient to render a breakdown diode BDconductive to establish a circuit through the primary winding 90 of thepulse transformer T2. Generally the capacitor C4 is fully charged duringeach negative half cycle. The circuit through the primary 90 isestablished from the plate C4a of the capacitor C4 through a junction85, across the capacitor C5, through the junction 53, 54, the primary90, anode and cathode electrodes of the diode D12, and to the plate C411through the breakdown diode BD, and junction 86. The capacitor 05provides a low impedance discharge path for the capacitor C4 so that thecurrent pulse through the primary 90 is in the form of a well definedspike.

It should be apparent that the breakdown diode is selected to breakdownand conduct in response to a predetermined voltage across the capacitorC4. The time required for the capacitor C4 to charge sufficiently torender the diode BD conductive depends on the impedance of the flame. Ifthe flame flickers or is blown out of contact with the electrode 87, theflame impedance is high and the capacitor C4 charges relatively slowlyrequiring a number of half cycles of the power supply to breakdown thediode BD.

The pulse transformer T1 transmits current pulses to a siliconcontrolled rectifier SCR4 which is coupled to the SCR2 so that when theSCR4 conducts the SCR2 and the SCR1 are rendered conductive to open thefuel valve. Current in the primary 90 of the transformer T1 induces acurrent pulse in the secondary winding 91 which is conducted to thecontrol electrode 92 of the SCR4. The pulse renders that SCR conductiveto establish a circuit from the junction 44 through a resistor R25, ajunction 93, anode and cathode electrodes of the SCR4, junctions 94- 96,a resistor R26, and to the terminal A through the junction 97. Aresistor R27 is connected across the control and cathode electrodes ofthe SCR4 between a junction 100 and the junction 95 to prevent that SCRfrom being turned on when undesired by transient voltages across theanode and control electrodes.

A diode D14 is connected between the junction 96 and the secondarywinding 91 of the transformer T1 with its cathode electrode connected tothe winding 91 to decouple the winding 91 from the SCR4 during collapseof the primary field which would otherwise result in the SCR4 turningoff. A filter capacitor C6 is connected between the junction 93 at theanode electrode of the SCR4 and a junction 101 connected to the terminalA. The capacitor 8 C6 prevents rapid voltage rises across the anode andcathode electrodes from rendering the SCR4 conductive when undesired.

The SCR4 conducts to drive the SCR2 when the SCR4 conducts. A signalcircuit is established from the junction 94 to the control electrode 51of the SCR2 through a diode D15, a resistor R30, a junction 102, a diodeD16, a resistor R31, and the junctions 62, 63. The described signalcircuit renders the SCR2 conductive which in turn drives the SCR1 toopen the valve 14.

A capacitor C7 is included in the circuitry to increase the sensitivityof the flame detecting circuitry to high impedance flames. The signalcircuit just described provides charging current for the capacitor C7whichis connected between the junction 102 and a junction 105 at thevoltage of the terminal A. The capacitor C7 has a rather large capacityand the plate C7a of the capacitor C7 is maintained positive withrespect to the plate C7b during operation of the SCR4.

Should the flame established at the burner 12 flicker or otherwise be ofrelatively high impedance, several negative half cycles of the powersupply may be required to produce a pulse in the primary of thetransformer T2. In that event the capacitor C7 discharges through thejunction 102, diode D16, resistor R31, junctions 62, 63, and to thecontrol electrode 51 of the SCR2, junction 46, resistor R2, junction 47to the negative plate C7b of the capacitor through the junction 105. Theresistor R31 is of relatively high impedance compared to the resistorR30. Thus the discharge rate of the capacitor C7 is relatively low incomparison to its charging rate. Discharging of the capacitor C7prefreably occurs over several cycles so that the SCR2 is maintainedconductive between low frequency pulses in the transformer T2.

When room air temperature reaches the preselected temperature, thethermostatic switch 25 opens interrupting the energization circuit forthe fuel control valve 14 and terminating the flow of fuel to the burner12. Opening of the thermostatic switch also resets the ignition trialcircuitry for a succeeding heating cycle. The trial circuitry is resetby discharging of the capacitor C1. The capacitor C1 is dischargedthrough the junction 67, the resistor R8, junction 55, the resistor R5,junctions 56, 57, 60, the resistor R12, the junction and to thecapacitor C1. Even though the capacitor C1 discharges, the transistor Q1is not rendered conductive since thermostatic switch 25 is opened andthe voltage levels at its anode and control electrodes are substantiallythe same. Thus when the contacts of the switch 25 are opened thetransistor Q1 is incapable of conducting pulses to the control electrodeof the SCR2 and since the flame is not present at the burner 12 theflame detecting circuitry is ineffective to provide operating signals tothe SCR2.

In the illustrated embodiment the capacitor C1 discharges relativelyslowly because the resistor R8 is large. This prevents excessiveaccumulations of unburned fuel in the furnace if ignition isunsuccessful due to malfunction and the user of the furnace attemptsfrequent ignition trials.

When the room air temperature is reduced sufficiently to cause reclosingof the contacts of the switch 25, the capacitor C1 is again connectedacross the terminals B, A through the contacts of the switch 25 and theignition trial circuitry 50 is rendered effective to open the gas valveand provide ignition arcs.

SELF-CHECKING FEATURES OF THE CIRCUITRY The present inventionadditionally provides safety circuitry which permits self-checkingoperation of the burner control circuitry upon each opening of thecontacts of the thermostatic switch 25. Without the safety circuitry,and if the SCR1 should short out, the control circuitry could operate ina seemingly normal fashion for an indefinite period of time. If duringthis period, and at a time when the contacts of the thermostatic switch29 were closed, the flow of gas to the furnace would for some reason bebriefly interrupted, the fuel valve 14 would be maintained open even inthe absence of flame at the burner. In such circumstances a fire orexplosion could result.

The self-checking function of the safety circuitry preventsmalfunctioning of the SCRI from going undetected. Furthermore, thesafety circuitry prevents opening of the fuel valve in response to anyflame simulating failure in the circuitry tending to maintain the valveopen when the thermostat is satisfied.

According to the present invention the safety circuitry provides a checkon the operation of the SCRl upon each opening of the contacts of thethermostatic switch 25. In this manner shorting of the SCRI results inrendering the circuit incapable of energizing the fuel valve in the samecycle of operation of the burner during which the malfunction occurs.Flame simulating malfunctions in the circuitry tending to open the gasvalve when the thermostatic switch contacts are opened also renders thecircuit incapable of opening the valve.

The safety circuitry includes a diode D20, and a relay coil 110connected between the junction 33 at the cathode electrode of the SCRland a junction 111 connected to the terminal A. In the preferredembodiment the safety circuitry is connected in parallel with the switchcontacts 34, 35 of the switch S1 and if the SCR1 is shorted out, openingof the thermostatic switch 25 results in the relay 110 being energizedthrough the SCRl, junction 31, diode D20, a junction 112, the relay coil110, a junction 113 and to the junction 111 connected to the terminal A.Energization of the relay 110 opens the normally closed contacts 34, 35and interrupts the energization circuit for the solenoid 15. Thesolenoid energizing circuit is thereafter maintained open regardless ofwhether the thermostatic switch 25 is closed.

As has been previously pointed out the terminals A, B and C are alwaysconnected across the power supply while the terminal D can bedisconnected from the supply by opening of the thermostatic switch 25.Hence even when the switch 25 is open the power supply voltage isimpressed across the terminals C, A and B, A, respectively.

Normally, of course, current does not flow between these terminals whenthe thermostatic switch is open. However should a component orcomponents of the system malfunction in a manner simulating the presenceof flame, the connection of the terminals A, B, C across the powersupply will cause such a flame simulating malfunction to occur. For thisreason any flame simulating failure in the circuitry Will falselyindicate the presence of flame at the burner at the first opening of thecontacts of the switch 25 after the failure occurs. Likewise, if theSCRl or the SCR2 should become short circuited, conduction of theshorted SCR will be maintained after the switch 25 opens. For thisreason the relay 110 is continuously energized when such malfunctionsoccur and when the switch 25 is opened thereby insuring that thesolenoid valve remains closed.

One mode of malfunction of an SCR is known as a gate-anode semishort.When such a malfunction occurs the impedance of the relay 110 couldcause chattering of the contacts 34, 35. A resistor R35 is connectedbetween the juntcions 112, 113 in parallel with the coil 110 so that theimpedance across the junctions 112, 113 is low enough to prevent relaychattering.

The resistance of the relay coil 110 and parallel resistor R35 is enoughso that when the SCRl is shorted as described most of the voltage fallsacross the relay and resistor and the solenoid cannot be pulled in. Thediode D prevents energization of the solenoid during negative halfcycles of the power supply if the malfunction of the SCRl is such thatfull wave current could otherwise be conducted. A diode D21 is connectedin parallel with the coil to maintain the contacts 34, 35 open duringnegative half cycles of the power supply.

When the room air temperature is reduced to a level sufiicient toreclose the thermostatic switch 25, the relay coil 110 is maintainedenergized because the contacts 34, 35 of the switch S1 are open andprevent establishment of an energizing circuit for the solenoid throughthe thermostatic switch. Thus, the fuel supply valve cannot be reopenedregardless of the condition of the contacts of the thermostatic switch.Fuithermore, the danger of fire or explosion resulting from unburned gasaccumulating in the furnace is eliminated.

In certain constructions of the control circuitry it might be possiblefor the terminal A to become disconnected from the power supply. In sucha case the terminal D could remain connected to the power supply(FIG. 1) and if the thermostatic switch 25 were closed the fuel valve 14could be energized through the SCRl, switch S1 and the thermostaticswitch. Furthermore in such circumstances the relay coil 110 could notbe energized in response to conduction of the 'SCRl in the absence of aflame.

Under such circumstances the filter capacitors 49 or C6 would render theSCRl conductive during positive half cycles of the power supply, therebyopening the gas valve and admitting unburned gas into the furnace. Adiode D22 connected between the junctions 115, 116 at the cathodeelectrodes of the diodes D1, D20, connect the capacitors 49, C6 to theterminal D to prevent the SCRI from conducting as described. Theconnection of the filter capacitors to the terminal D is traced from thecapacitors to the junction 120, through the junctions 47, 121, 105, 111,113, the diode D21, junctions 112, 115, the diode D22, the junction 116and to the terminal D through the switches :51, 25. The path through thediodes D21, D22 has a low impedance and thus triggering of the SCRl isinsured against.

Although the invention has been described in its preferred forms with acertain degree of particularity, it is understood that the presentdisclosure of the preferred forms has been made only by way of exampleand that numerous changes in the details of the constructions and thecombinations and arrangements of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

What is claimed is:

1. A system for controlling the temperature of a medium heated by a fuelburner including:

(a) an electric power supply;

(b) an electrically energized fuel control valve which is energizable toan open position wherein fuel flows to said burner and a deenergizedclosed position;

(c) circuitry for energizing said valve comprising;

((1) switching circuitry including a switching element connected inseries with said valve;

(e) a thermostatic switch comprising contacts having a closed positionwherein an energization circuit is establishable through said contacts,valve and switching element, and an open position wherein saidenergization circuit is opened; and,

(f) means responsive to conduction of said switching element when saidcontacts of said thermostatic switch are open for preventing completionof said valve energizing circuitry upon closing of said contacts of saidthermostatic switch.

2. A system as defined in claim 1 and further including flame detectingcircuitry coupled to said switching circuitry and operative to maintainsaid switching element conducting when flame is present at said burnerand said thermostatic switch is closed.

3. A system as defined in claim 2 and further including ignition trialcircuitry coupled to said switching circuit and operable to render saidswitching element conductive for a brief interval subsequent toconnection of said valve energization circuit across said power supplythrough said thermostatic switch.

4. A system as defined in claim 3 wherein said ignition trial circuitryis ineffective to render said switching element conductive at the end ofsaid interval, and further including enabling circuitry operative torender said ignition trial circuitry effective after a second intervalsubsequent to said first interval.

5. A system as defined in claim 4 and further wherein said ignitiontrial circuitry includes a semiconductor switch having a controlelectrode connected to a point in a charging circuit and an outputcircuit coupled to said switching circuitry, said charging circuitproducng a first range of a voltage at said point for maintaining saidsemiconductor in a conductive state during said interval and a secondrange of voltages at said point for preventing conduction of saidsemiconductor after said interval.

6. A system as defined in claim 5 wherein said enabling circuitryincludes a discharge circuit for changing the range of voltages at saidpoint to said first range of said second range when said thermostaticswitch is open.

7. A system as defined in claim 2 wherein said flame detecting circuitryprovides periodic flame signals to said switching circuitry in thepresence of flame at said burner and further including a chargingcircuit coupled to said detecting circuit and to said switchingcircuitry, said charging circuitry operable to condition said swtchingcircuitry to maintain said valve open over a range of frequency of saidflame signals.

8. A system as defined in claim 1 wherein said means includes a normallydeenergized circuit element connected in circuit with said switchingcircuitry, said circuit element energized in response to conduction ofsaid switching element when said thermostat contacts open,

and further including a second switching element opi.

erated from said circuit element to prevent energization of said valvewhen said thermostat contacts reclose.

9. A system as defined in claim 8 wherein said circuit element comprisesa relay and said second switching element comprises relay contactsactivated by said relay, said relay contacts connected in series withsaid thermostat contacts and said first switching element and having anopen position preventing conduction of said first switching elementthrough said thermostat contacts.

10. A system as defined in claim 9 wherein said relay contacts areopened to establish a circuit through said valve, said first switchingelement and said relay, said relay maintained energized when said firstswitching element conducts after said thermostatic contacts open, saidrelay having an impedance sufliciently large to prevent opening of saidvalve.

11. A flame monitoring system comprising:

(a) an electrical power supply;

(b) an electrically energizable fuel control valve including anelectrically operated actuator which is energized to open said valve anddeenergized to close said valve;

(c) switching circuitry having a conductive and a nonconductivecondition, said switching circuitry establishing an energizing circuitfor said actuator when in one of said conditions and preventingenergization of said actuator when in its other condition;

(d) a condition responsive switch in said energizing circuit having anonconducting condition to interrupt said circuit and a conductingcondition to permit energization of said actuator;

(e) flame responsive signal producing circuitry for operating saidswitching circuitry to said one condition when a flame is established;and,

(f) safety circuitry operative to deenergize said actuator and closesaid valve in response to any flame simulating malfunction in the systemtending to maintain said switching circuitry in said one condition whensaid condition responsive switch becomes nonconducting;

(g) said safety circuitry maintaining said actuator deenergizedthereafter regardless of the condition of the condition responsiveswitch.

12. A system as defined in claim 11 wherein said system includes aburner for heating a medium, said condition responsive switch comprisinga thermostat having a thermally responsive element in heat exchangerelationship with said medium and operating said switch between saidconditions in response to temperature of said medium.

13. A flame monitoring system as defined in claim 11 wherein said signalproducing circuitry includes circuit elements for producing periodicelectrical pulses in response to said flame, said switching circuitryincluding a semiconductor switch in series with said actuator and saidcondition responsive switch, said semiconductor switch renderedconductive to energize said control valve actuator in response tosignals produced by said signal circuit when said condition responsiveswitch is conductive.

14. A system as defined in claim 13 wherein pulses produced by saidsignal producing circuitry have a frequency which decreases as theimpedance of said flame increases, and further including a chargingcircuit coupled between said signal circuit and said switchingcircuitry, said charging circuit operable to maintain said switchingcircuitry effective at relatively low pulse signal frequencies so thathigh impedance or flickering flame can be maintained and monitored bysaid system.

15. Burner control circuitry for operating an electrically actuated fuelvalve for a burner comprising:

(a) a power supply;

(b) an electrically actuated fuel valve operable between open and closedpositions;

(c) an energizing circuit for the fuel valve established across saidpower supply between first and second terminals;

(d) said energizing circuit comprising a first switch operable between aconductive condition and a nonconductive condition, said first switchoperable to one condition for permitting actuation of said fuel valve inresponse to the presence of flame and a second switch operable between aconducting condition and a nonconducting condition, said second switchoperable to one condition for permitting actuation of said valve inresponse to sensed temperaure;

(e) a second circuit in series with said fuel valve and said firstswitch and connected to said power supply through a third terminal; and,

(f) means in said second circuit for interrupting said first circuit inresponse to said first switch being in said one condition when saidsecond switch is in its other condition, said means maintaining saidfirst circuit interrupted thereafter.

16. Circuitry as defined in claim 15 wherein the impedance of saidsecond circuit is of a magnitude which is sutficient to preventactuation of said fuel valve to the open position when said secondcircuit conducts.

17. Circuitry as defined in claim 15 wherein said first switch is agated semiconductor switch, and further including circuitry connected toa gate electrode of said switch for rendering the switch conductive.

18. A flame monitoring system comprising:

(a) an electrical power supply;

(b) an electrically energizable fuel control valve including anelectrically operated actuator which is energized to open said valve anddeenergized to close said valve;

(c) switching circuitry effective to establish an energizing circuit forsaid actuator;

(d) a condition responsive switch in said energizing circuit having anonconducting condition to interrupt said circuit and a conductingcondition to permit energization of said actuator;

(e) flame responsive signal producing circuitry for rendering saidswitching circuitry effective when a flame is established;

(f) safety circuitry operative to deenergize said actuator and closesaid valve in response to any flame simulating malfunction in the systemtending to maintain said valve open when said condition responsiveswitch becomes nonconducting; and,

(g) said safety circuitry comprising a relay connected in parallel withsaid condition responsive switch and in series with said actuator, saidrelay being energized only when said condition responsive switch isnonconducting and said switching circuitry is effective to establishsaid actuatorenergizing circuit and operative when energized to preventenergization of said actuator regardless of the conductive condition ofsaid condition responsive switch.

19.-Burner control circuitry for operating an electrically actuated fuelvalve for a burner comprising:

(a) a power supply;

(b) an electrically actuated fuel valve;

(c) an energizing circuit for the fuel valve establishable across saidpower supply between first and second terminals;

((1) said energizing circuit comprising a first switch permittingactuation of said fuel valve in response to the presence of fiame and asecond switch permitting actuation of said valve in response to sensedtemperature;

(e) a second circuit in series with said fuel valve and said firstswitch and connected to said power supply through a third terminal; and,

(f) means in said second circuit for interrupting said first circuit inresponse to conduction of said first switch when said second switch isopen and preventing continued energization of said fuel valve;

(g) said means comprising a relay connected between said first switchand said third terminal, said relay having contacts in said firstcircuit between said v 14 first and second switches, said contactsopening in response to energization of said relay.

20. Burner control circuitry for operating an electrically actuated fuelvalve for a burner comprising:

(a) apower supply;

(b) an electrically actuated fuel valve;

(c) an energizing circuit for the fuel valve establishable across saidpower supply between first and second terminals;

(d) said energizing circuit comprising a first switch permittingactuation of said fuel valve in response to the presence of flame andasecond switch permitting actuation of said valve in response to sensedtemperature;

(e) a second circuit in series with said fuel valve and said firstswitch and connected to said power supply through a third terminal;

(f) means in said second circuit for interrupting said first circuit inresponse to conduction of said first switch when said second switch isopen and preventing continued energization of said fuel valve;

(g) said first switch is a gated semiconductor switch and furtherincludes circuitry connected to a gate electrode of said switch forrendering the switch conductive; and,

(h) further including a unidirectional conductor between said first andsecond circuits for transmitting false gating signals around said firstswitch when said second switch is closed and said third terminal isdisconnected from the power supply.

References Cited UNITED STATES PATENTS 3,447,880 6/1969 Potts et a1.431--27 EDWQRD G. FAVORS, Primary Examiner U.S. Cl. X.R. 43 1--24 66

